This invention is generally directed to metal components which are exposed to high temperatures. More specifically, it relates to methods for protecting the metal components from the effects of high temperature and other extreme conditions. Compositions which help to provide that protection are also described herein.
Various metal components must be able to operate in a high-temperature environment. As one example, cobalt- or nickel-based superalloys are used for sections of turbine engines, and must be able to withstand temperatures in the range of about 650° C.-1200° C. These types of alloys contain aluminum, which is a key component for the precipitation-strengthening of the material.
If superalloys are exposed to an oxidizing temperature for an extended period of time, they can become depleted in aluminum. The depletion generally occurs in the surface region of the alloy. Aluminum depletion can be especially severe if the superalloy component is subjected to elevated temperatures, and/or repeated temperature cycling. Since loss of aluminum can damage the superalloy component, different techniques have been developed for compensating for that loss.
One common technique for increasing aluminum content in the surface region of a metal component is referred to as “aluminiding” or “aluminizing”. A specific example is the pack aluminiding process, in which the substrate is immersed within a pack containing a coating element source, filler material, and a halide activator. At elevated temperatures (usually in the range of about 700-750° C.), the reaction mixture releases an aluminum-rich vapor which condenses on the substrate surface. The condensed aluminum-based material diffuses into the substrate during a subsequent heat treatment.
Another method to deliver aluminum to the substrate involves the use of a slurry which contains the aluminum. The slurry can be applied to the substrate by various techniques. The slurry is then heated to remove the volatiles, and to diffuse molten aluminum into the substrate at high temperatures. Slurries are often desirable because they can be prepared economically, and can be easily applied to the substrate.
However, aluminum-containing slurries usually require the use of chromates or dichromates. These materials are considered toxic. In particular, hexavalent chromium is also considered to be a carcinogen. When compositions containing this form of chromium are used (e.g., in spray booths), special handling procedures have to be very closely followed, in order to satisfy health and safety regulations. The special handling procedures can often result in increased costs and decreased productivity.
The aluminum-containing slurries also may contain phosphate materials, such as phosphoric acid. The phosphate materials serve as a binder in the composition. However, materials like phosphoric acid sometimes attack the aluminum metal in the slurry composition, rendering it thermally and physically unstable.
Aluminum-containing compositions which are free of chromates and phosphate materials have been described in the literature. For example, U.S. Pat. No. 6,224,657 (Myers et al) discloses bonding compositions which are free of hexavalent chromium. The compositions are based in part on the use of trivalent chromium, Cr+3. Moreover, U.S. Pat. No. 6,368,394 (Hughes et al) describes an aluminum-containing, chromate-free composition based on water, phosphate ions, borate ions, and aluminum ions.
However, chromate-free slurry compositions are sometimes physically and/or chemically unstable over the course of several hours, or even after several minutes. This can be an especially serious problem in the case of aqueous slurries which allow free reaction between aluminum particles and water. The compositions can generate excessive amounts of gasses, such as hydrogen. Moreover, they can thicken or solidify relatively quickly, making them very difficult to apply to a substrate, e.g., by spray techniques. It may also be difficult to store the compositions. Furthermore, the phosphate component in some of these compositions may be wholly or partially converted to phosphoric acid. The phosphoric acid can attack the aluminum metal in the composition (especially in the absence of chromates), rendering the aluminum unstable.
In view of the state of the art, new compositions which are useful for aluminizing metal substrates would be of considerable interest. The compositions should be capable of incorporating as much aluminum as necessary into the substrate. They should also be substantially free of chromate compounds—especially hexavalent chromium. (In some preferred embodiments, the compositions should also contain relatively low levels of phosphoric acid, e.g., less than about 10% by weight).
It would also be desirable if the aluminizing compositions were chemically and physically stable for extended periods of use and storage. They should also be capable of being applied to the substrate in a variety of ways, such as spraying or brushing. Moreover, the processes involved in using the aluminizing compositions should generally be compatible with other techniques which might be used to treat a particular metal substrate, e.g., a superalloy turbine component.